A power cylinder assembly of an internal combustion engine generally includes a reciprocating piston disposed within a cylindrical cavity of an engine block. One end of the cylindrical cavity is closed while another end of the cylindrical cavity is open. The closed end of the cylindrical cavity and an upper portion or crown of the piston defines a combustion chamber. The open end of the cylindrical cavity permits oscillatory movement of a connecting rod, which joins a lower portion of the piston to a crankshaft, which is partially submersed in an oil sump. The crankshaft converts linear motion of the piston (resulting from combustion of fuel in the combustion chamber) into rotational motion.
The power cylinder assembly typically includes one or more piston rings and a cylindrical sleeve or cylinder liner, which is disposed within the engine block and forms side walls of the cylindrical cavity. The piston rings are disposed in grooves formed in the lateral walls of the piston, and extend outwardly from the piston into an annular space delineated by the piston wall and the cylinder liner. During movement of the piston within the cylindrical cavity, the piston rings bear against the cylinder liner. The piston rings have at least two functions. First, they inhibit gas flow from the combustion chamber into the oil sump through the annular space between the piston and the cylinder liner. Second, they minimize oil flow from the oil sump into the combustion chamber.
Piston rings generally must survive extreme temperatures and pressures resulting from the combustion cycle. Accordingly, the outer surface of a piston ring that bears upon the cylinder liner or bore surface often includes a hard surface coating, or is otherwise treated to create a hardened outer surface that is more durable than an untreated surface. Coatings applied via spraying are inherently difficult to apply accurately, and the piston rings may be masked in some form to prevent sprayed coatings from adhering to surfaces other than the intended outer piston ring surface.
Increases in fuel economy and emission requirements for engines have made surface treatments more desirable for surfaces other than the outermost piston ring surface. However, similar difficulties in masking the desired areas for treatment are inherent, and other piston ring surfaces cannot be masked using the same stacking approach as for outer piston ring surface treatments.
Accordingly, some known power cylinder assemblies include piston rings with a chromium plated lateral side, i.e., the side of the ring positioned to interface with piston ring groove surfaces. However, known chromium coated rings suffer from inadequate layer thickness, typically being approximately 5-10 microns (also referred to as micrometers). Thus, although coated with typically 5-10 microns, during the life of the piston frictional wear can result in early life failure of the piston, due to the extended cyclical loading of the ring within the groove. That is, these chromium layers can wear through before the end of the service life of a piston ring, which can lead to early life failure of the engine.
Additionally, chromium plated coatings that are greater than approximately 5-10 microns can result in a significant amount of waviness that can result in blow-by of combustion products or inadequate scraping action. Also, chromium plating greater than approximately 5-10 microns typically includes nodules that remain after the plating process, which can lead to piston groove wear and early life failure of the engine, as well. The propensity for generating nodules increases as the thickness of the plating increases, and thus layer thickness generally may not be increased without a corresponding increase in surface roughness.
As such, there is a need to improve piston ring designs.